The present invention relates generally to dielectric filters, and, more particularly, to a filter assembly having a dielectric filter and an electromagnetic wave-absorptive shield affixed thereto which is permitting of surface mounting of the filter upon a substrate.
Advancements in the field of radio electronics have permitted the introduction and commercialization of an ever-increasing array of radio communication apparatus. Advancements in electronic circuitry design have also permitted increased miniaturization of the electronic circuitry comprising such radio communication apparatus. As a result, an ever-increasing array of radio communication apparatus comprised of ever-smaller, electronic circuitry has permitted the radio communication apparatus to be utilized more conveniently in an increased number of applications.
A radio transceiver, such as a radio transceiver utilized in a cellular, communication system, is one example of radio communication apparatus which has been miniaturized to be utilized in an increased number of applications. Additional efforts to miniaturize further the electronic circuitry of such radio transceivers, as well as other radio communication apparatus, are being made. Such further miniaturization of the radio transceivers will further increase the convenience of utilization of such apparatus, and will permit such apparatus to be utilized in further increased numbers of applications.
Pursuant to such efforts to miniaturize further the electronic circuitry comprising radio transceivers, as well as other radio communication apparatus, size miniaturization of the electronic circuitry comprising such is a critical design goal during circuit design.
Dielectric block filters, comprised of a ceramic material, frequently comprise a portion of the circuitry of such radio transceivers. Dielectric block filters are advantageously utilized as such filters exhibit good filter characteristics at frequencies at which such transceivers usually are operative.
To form a filter of a block of dielectric material, holes are molded, or otherwise formed, to extend through the dielectric block, and sidewalls defining such holes are coated with an electrically-conductive material, such as a silver-containing material. The holes formed thereby form resonators which resonate at frequencies determined by the lengths of the holes.
Typically, substantial portions of the outer surfaces of the dielectric block are similarly coated with the electrically-conductive material. Such portions of the outer surfaces are typically coupled to an electrical ground.
Spaced-apart portions of a top surface of the dielectric block are also typically coated with the electrically-conductive material which is electrically isolated from the electrically-conductive material coated upon other outer surfaces of the dielectric block. Adjacent portions of the electrically-conductive material coated upon the top surface become capacitively coupled theretogether. Additionally, such portions capacitively load respective ones of the resonators.
The resonators, due to the electromagnetic coupling between adjacent ones of the resonators, the portions of the top surface of the block (due to capacitive coupling), and the capacitive loading of the resonators together define a filter having filter characteristics for filtering a signal applied thereto.
In actual dielectric block filters, electromagnetic intercoupling exists not only between adjacent resonators of the filter, but additionally, between nonadjacent ones of the resonators. The intercoupling between the nonadjacent ones of the resonators is generally undesired, and, frequently, some type of electromagnetic wave-absorptive material configured to form a shield is positioned proximate to top surfaces of such dielectric block filters. Such shields are operative to minimize the undesired intercoupling between nonadjacent resonators. To operate properly, such shields are grounded to the same electrical ground potential as the electrical ground to which the dielectric block filters are connected. And, most simply, the shields may be affixed, or otherwise connected, directly to the filters.
However, when connected to a dielectric block filter, the shield alters the filter characteristics of the filter.
After construction of a dielectric block filter, the filter is tuned by removing portions of the coating of the electrically-conductive material. Such tuning corrects for manufacturing variances, and is typically performed to alter slightly the filter characteristics of the filter. Conventionally, the filter is placed in a supportive fixture, the filter characteristics of the untuned filter are determined, and then the filter is tuned to be of desired filter characteristics. Once the filter has been tuned by such a process, the filter is removed from its supportive positioning in the supportive fixture, a shield is affixed to the filter, and the filter is placed upon a circuit board and connected to an electrical circuit to which the filter then forms a portion. But, as noted hereinabove, the shield alters the filter characteristics of the filter; hence, the filter characteristics of the filter, once the shield is affixed thereto, differs somewhat with the filter characteristics of the filter, as originally tuned.
Such variance between the tuned, filter characteristics and the filter characteristics of the filter after affixation of the shield to the filter can result in undesired performance of a circuit to which the filter forms a portion.
What is needed, therefore, is a shield for a dielectric filter, and a filter assembly including such, which may be affixed to the dielectric filter prior to tuning thereof.
Automation of circuit assembly is effectuated by the use of reflow solder techniques. Dielectric block filters which may be surface-mounted upon a circuit board permit affixation of such filters to the circuit board by a reflow solder technique. Use of dielectric filters which may be surface-mounted therefore advantageously facilitates automation of circuit assembly.
For a filter to be surface-mountable, the face surface of the dielectric block filter which seats upon the circuit board must be flat. Accordingly, a shield which is affixed to the dielectric block filter must be of a construction permitting affixation thereof to the filter while still permitting the bottom face surface of the dielectric block filter to be of a flat configuration.
What is further needed, therefore, is a filter assembly comprised of a dielectric block filter and a shield affixed thereto wherein the filter, after affixation of the shield thereto includes a flat seating surface permitting seating of the filter upon a circuit board, thereby to permit affixation of the filter assembly to an electrical circuit disposed upon the circuit board by a reflow solder technique.